CA2081459A1 - Swirl ring and flow control process for a plasma arc torch - Google Patents

Abstract

Abstract Of The Disclosure A process and apparatus for reducing wear of an electrode in a plasma arc torch involves providing a substantial pressure drop in a flow of plasma gas from the torch to a plasma arc chamber defined by the electrode and a nozzle. The pressure drop is immediately before the plasma arc chamber. There is also a small, localized supply of the plasma gas located between the pressure drop and the plasma chamber. The supply is sufficient to stabilize the arc when the gas flow is cut off upstream of the pressure drop and the arc current is cut off very shortly thereafter. The pressure drop is small enough to permit an adequate flow of plasma gas to the plasma arc chamber during normal operations, but large enough to isolate residual gas in the torch from the plasma arc chamber when the arc current is cut off. The apparatus is preferably a swirl ring with an annular pre-chamber fed by a set of choke holes to feed gas to the chamber and create the pressure drop. A set of swirl holes feed gas from the pre-chamber to the plasma arc chamber.

4436m

Description

SWIRL RING A~D FLOW CONTROL PROCESS
FOR A PLASMA ARC TORCH

Reference to Related Applications This application is a continuation-in-part of U.S. application Serial No. 07J513,780 filed on Aprll 24, 1990.

Backqround Of The Invention This invention relates in general to plasma ar~ cutting and welding processes and apparatus. More specifically, it relates to a process and apparatus for reducing electrode wear, particularly in high power torches.
Plasma arc torches have a wide variety of applications such as the cutting of thick plates of steel and the cutting of comparatively thin sheets of galvanized metal commonly used in heating, ventilating and air conditioning (HVAC) systems. The basic components of a plasma arc torch include a torch body, an electrode (cathode) mounted within the body, a nozzle (anode) with a central exit orifice, a flow of an ionizable gas, electrical connections, passages for cooling, and arc control fluids, and a power supply that produces a ~ilot arc in the ga~, typically between the electrode and the nozæle, and then a plasma arc, a conductive flow of the ionized gas from 2 ~ 9 the electrode to a workpiece. The gas can be non-reacti~e, e.g. nitrogen, or reactive, e.g. oxygen or air.
Various plasma arc torches of this general type are described in U.S. Patent Nos. 3,641,304 to Couch and Dean, 3,833,787 to Couch, 4,203,022 to Couch and Bailey, 4,421,970 to Couch, 4,791,268 to Sanders and Couch, 4,816,637 ~o Sanders and Couch, and 4,861,962 to Sanders and Couch, all commonly assigned with the present application. Plasma arc torches and related products are sold in a variety of models by Hypertherm, Inc. of Hanover, New Hampshire. The MAX
10~ brand torch of H~pertherm is typical of the medium power torches (lO0 ampere output) using air as the working gas and useul for both plate fabrication and HVAC applications. The HT 400 brand torch is typical of the high power torches (26~ amperes) often using oxygen as the work;ng gas. High power torches are t~pically water cooled and used to pierce and ~ut thick metal sheets, e.g. 1 inch thick mild steel plate.
In all plasma arc torches, a common and heretofore unsolved problem has been a substantial wear of the electrode, particularly whe~ th~ electrode is used with reactive gases such as oxygen or air.
(Improved wear, other conditions being the s~me, i5 observed when using non-r~acti~e gases such as nitrogen ~r argon as the plasma ga~, but the performance using pure oxygen is superior at least w~en used to cut certain materials such as mild steel. Similarly, air is superior to pure oxygen with respect to wear, but there is again a performance trade off.) As an example of this wear problem, the 208~5~

standard electrode for the MAX 100 brand torch of .
Hypertherm, Inc. shows wear as a generally concave pit on the lower end of the electrode, or more precisely, on an emitting element o hafnium mounted on the electrode. On average a wear dep~h of about 0.025 inch is observed in such a Hypertherm brand electrode after 120 cut cycles operating with oxygen or air.
The wear results of commercially available units of others, as measured by Hypertherm, Inc., are typically worse. For the MAX 100 brand ~orch, when the wear produces a pit depth of 0.060 inch or more, Hypertherm, Inc. recommends that the electrode b~
replaced. In ordinary use, the electrode of a plasma arc cutting torch operating with reactive gases ~ypically reguires replacement after 0.5 to 2 hours of use depending strongly on the number of on-off cycles. Wear considerations are significant not only because they necessitate the repeated replacement of a component, but also because they limi~ the maximum power that can be applied to a given torch. With particular reference to the present invention, it has proven especially difficult to control electrode wear in high current torches, e.g. the water cooled torches sol~ by Hypertherm, Inc. under the trade designation HT ~00 and PAC 500, respectively.
In plasma arc cutting, it is also impor~ant to note that the ~uality o the cut i~ highly dependent on the flow pattern of the gas in a plasma chamber, defined at least in part as the region between the electrode and the nozzle. In particular, a swirling flow produced by injecting the gas tangentially into the plasma chamber has been found to . . .

2~8~9 be essential to produce a high quality cut. A
swirling gas flow pattern is also important in stabilizing the plasma arc so ~hat it exits the ~orch to attach to and cut the workpiece, but does not contact the torch nozzle itself. The nozzle is the principal component that is damaged by the arc when the arc is not well controlled. Heretofore ~he swirling gas ~low is o~ten produced mainly by a swirl ring Shat has angled holes formed in the ring that feed a swirling gas flow to the plasma chamber. The aforementioned U.S. Patent No. 4,861,926, also describes a swirling secondary cooling gas flow passing between the nozzle and a surrounding shield member to assist in the arc stablli2ation.
Another design consideration is the very high temperatures of the plasma, e.g. greater than 10,000C. These temperatures introduce corresponding changes in the gas properties such as its density and viscosity. These considerations are significant on start up and cut-off. On start up the arc rapidly heats the gas which significantly decreases the gas density exiting the nozzle orifice. This present~ ~he situation where the gas flow is choked in the nozzle orifice region. This choking is, in general, advantageous during cutting since it restricts the flow of gas rom plasma chamber to atmosphere and thereby maintains a~ elevated gas pressure level in the plasma chamber that constricts the arc. This leads to an improved cut. A typical gas pressure in ~he plasma chamber to achieve these beneficial effects in a medium to high power torch is about 40 psig. On cut-off of the arc current, the situation reverses and kosl~s there is a tendency for the gas in the plasma chamber ~o cool and blow out of the chamber very suddenly.
It is common industry practic~ to use hanium or 2irconium as the ca~hodic emitter inser~ in the electrode. Hafnium, a~ o~ today, is the be~t choice for the cathodic emitting el~ment when cutting with a reac~ive gas plasma. It exhibi~s the lea~t wear of all other materials tried ~or this application~ but is more costly than other materials. These electrodes never~heless reguire frequent replacement. Lower wear has been as~ociated with lower current level~, but at some po;nt the reduction in performance associated with a reduced operating current becomes too great.
Cooling the electrode has also been used to increase electrode life, whether by way o~ a gas ~low or water flow placed in good thermal communication with ~he electrode. Howe~er, water cooling is expe~sive, cumbersome and is not desirable for low current units, e.g. those rated below 100 amperes. Air cooling is less efficient and limits the maximum operating current of the torch, even one carrying a comparatively low current. Therefore, to date, the only practical solution to the electrode wear problem has been to replace the entire electrode again and again, despite the clear economic disadva~tage~ o this approach.
It is therefore a principal object of the present i~vention to reduce the wear on the electrode of a plasma arc torch significantly and ther~by extend it~ life.
Ano~her ~rincipal object of thi~ invention i~
to reduce electrode wear ~nd thereby allow operation ~81~

at higher current levels than are presently easible, even when operating with reactive gases.
Still another pri~cipal object of thi~
invention is to provide a swirl ring which in addition to producing a swirling output of ~he gas also:
controls the gas flow to the plasma chamber and the distribution o the gas i~ the plasma chamber.
Another object of the invention i~ to achieve a better cut guality than has heretofore been possible by allowing a greater level of swirl.
Another o~ject of the invention is to provide the foregoing advantages while using standard electrode and nozzle constructions and without any significant increase in the incidenc~ of damage to torch parts such as nozzle gouging.
Yet another object of the i~vention i~ to provide the foregoing advantages for existing plasma arc torch systems using only comparatively ~imple and inexpensive modi~ication~.
A still urther object is to provide the foregoing advantages at a favorable cost vf manufacture and operation.
Summary of the Invention -A plasma arc cutting torch, particularly o~e using a reactive ga~ and employed in cutting me~allic materials, has a torch body that mounts an ~l~ctrode and a nozzle in a spaced relationship to define a plasma charnber therebetween. An ionizable ga~ is fed by an inlet system which can consist of tubes, passages and/or cham'Qers to and through l:he torch body .. . ... . . . . .

`
r 2 (~ 81 A~ ~; 9 to a swirl ring mounted in the torch hody. The swirl ring feeds the gas to the plasma chamber in a swirling flow pattern where it is ionized and exits the ~orch via a central exit orifice formed in the nozzle. The torch also includes standard electrical connections to a power supply and an electrical controller ~o initiate a pilst arc în the yas in the ch~mber and th~n transfer ~he arc to a workpiece for cutting or other operations.
The swirl ring of th~ present invention is an annular member mounted in the torch body adjacent the electrode, the nozzle and the plasma chamber. It is typically secured between the torch body and the nozzle. The swirl ring includes a pre-chamber, preferably an annular recess extending around the outer surface of the swirl ring. A set of shoke passages are formed in an upstream face of the swirl ring to conduct gas from the gas inlet system in the torch body to the pre-chamber. The number, distribution and size of these choke passages restrict the gas flow and thereby introduce a gas pressure drop on the feed line immedia~ely before the plasma chamber. The plasma gas exits the pre-chamber to the plasma chamber via a set of angled holes found in the downstream face o the ~wirl ring. In the preferred form the swirl ring also include~ a set of radial outle~ holes leading ~rom the pre-chamber to the plasma chamber.
The swirl ring includes annular recess~s for o-rings that seal the ~wirl ring to the torch body at its upstream face, to the nozzle at its downstream face, and to a current ring or other surrounding torch 2~4~9 component at its outer lateral face. In the preferred form the pressure drop along the gas inle~ hole~ to the swirl ring is about 32 psi when the plasma reaches steady state and the gas pressure in the plasma chamber is about 40 psi.
This gas control swirl ring is,particularly useful when the electrode wear is further controlled by altering the mass flow rate of the gas, and/or its flow pattern, immediately before and immediately aftèr the step of cutting off of the current to the torch.
The mass flow rate is reduced by either c~osing off or reducing the gas flow to the plasma chamber~ This mass flow rate reduction is timed to o~cur within a few hundred milliseconds before the curr~n~ cut off, and preferably continue aft~r cut off~ The process step of cutting the arc current can be accomplished by a sudden step function of time or a gradual ramp function of time~ The reduction in the gas flow may be coupled with a venting of the plasma chamber to atmosphere to facilitate a more rapid change in the gas flow pattern in the plasma chamber.
The present swirl ring with gas control invention can also be viewed as a process characteri~ed principally by the creation of a pressure drop i~ the gas feed line to the plasma chamber at a point immediately be~ore the plasma chamber. This pressure drop i~ sufficiently low that at steady ~tate operation the arc is stable and well controlled. The pressur8 drop is also sufficien~ly ~igh that when the gas flow to th~ torch is cut off~
the gas flow through the plasma chamber i~ al~o cut off rapidly. This limits a sudden and large outrush .

2~81~9 g of gas from the plasma chamber as the arc extinguishes and the plasma gas cools. The process includes providing a small localized source of gas immediately adjacent the plasma chamber ~o that there is sufficient gas available to the plasma chamber to maintain control over the arc after the gas flow is cut off, but before the arc is fully extinguished and could o~h0rwise damage the torch. These proce~s features can al~o be achieved by placing the aforementioned flow restrictions and pre-chamber in a torch component or components other than th~ swirl ring, such as a portion of the torch body adjacent the plasma chamber.
These and other features and objects of this invention will be more fully understood from the detailed description which should be read in light of the accompanying drawings.

Brief Description Of The Drawinqs Fig. lA is a simplified schematic view o a prior art typical plasma arc torch connected to ~ gas - flow control that controls the ga~ flow to the torch to reduce electrode wear;
Fig. lB is a view in ~orizontal sectio~ alo~g the li~e lB-lB in Fig. lA;
Fig~ lC is a ~implified schematic view of a plasma arc cut~ing fiy~tem using the tor~h shown in Figs. lA and lB;
Fig. ~D is a six graph timing diagram of the gas flow alteration in relation to the cut-off arc - current that has been found to extend electrode life;

14~

Figs. 2A, 2B and 2C correspond to Figs. lA, lB and lC and show an alternative arrangement for electrode wear reduction utilizing a valved vent in combination with a valved gas feed;
Figs. 3A, 3B, 3C, and 3D correspond to Fig~.
2A - 2C, Figs. 3B and 3C corresponding to Fig. 2B, and ~how an alternative arrangement utilizing axial and radlal inlet hole sets in a swixl ring to establish either swirled or axial gas flow pattern~ in the plasma chamb~r;
Fig. 4 is a simplified schematic view c~rrespondin~ to Fig. lC showing flow control mechanisms in the gas supply line to provide a preflow and.xamp up of the flow on ~tart up as well as a ramp down on cut of f;
Fig. 5 i~ a five graph timing diagram of the gas flow alteration on start-up;
Fig. 6 i~ a simplified view in vertical section of a swirl ring with a gas control function according to the present invention mounted in a plasma arc torch of the general type ~hown in Figs. lA, lC, 2A, 2C, 3A, 3D and 4, with ~he upper portions of the torch body broken away and other components, such as a retaining cap, omit'ted;
Fig. 7 is a bottom pla~ view of the swirl ring only sho~m in Fig. 6 taken along the line 7-7; and Fig, 8 is a graph plotting the gas pressure i~ the plasma chamber of the torch show~ in Fig. 6 as a function of ~ime during one cycle of operation and also plotting the arc current carried by the plasma o~er the same cycle of operation.

~D 2 ~

De~ailed Description Of_The Preferred Embodiments Figs. ~A and lB ~how in simplified schematic form a typioal pla~ma arc cutting torch 12 such as any of a varie~y of models of ~orches sold by Hypertherm, Inc. The i~mediately follpwing description is therefore intended ~o illus~rate the basic principles and elements common to pla~ma arc cutting torches of this type, bu~ not to describe construc~ion details o~
any particular torch model. The torch ha~ a body indicate~ generally at 14 which typically is cylindrical with an exit orifice 16 at one end, th~
lower end 12a as shown, through which a plasma arc 18, an io~ized gas jet, passes and attaches to a metal workpiece 20 which is being cut. The gas can be non-reactive, such as nitrogen or a noble gas, but it can also be reactive, such as oxygen or air. A
significant advantage of the swirl xing a~d associated gas control process of the present invention is that the torch can operate with a reactive gas and nevertheles~ have dramatic impro~ement in electrode wear even when operating at high power levels.
The torch body 14 supports a cathode 22, commonly referred to as an electrode, which is typically made of copper and has a generally cylindrical configuration. An emitting element 24 i~
press fitted in~o the lower end ~ace of the electrode 22. ~he electrode and the emitting element are sentered within the torch bod~ and aligned with the exi~ orifice 16. When using a reactiv~ gas, the insert i8 typically formed of hafnium or zirconium.
The body 14 al~o mounts a nozzle 26 with a central .. . . . .. ..

208~9 nozzle oriice that defines the torch QXit orifiCQ
16. The nozzle is ~paced rom the electrode. A swirl ring 28 of conventional construction and having a ~et o radially off~et gas di~tribution hol~s 30 is replaceably mounted in the torch body. It is positioned to fesd a flow of the pla~ma gas ~rom the holes 30 to a pla~ma arc chamber 32 defin2d, a~ ~hown, by the electrode, the nozzle and the swirl ring.
With this prior art conventional swirl ring the offset of the holes, best seen in Fig. lB, introduces a tangential ~elocity component to the gas flow through the chamber 32 causing it to ~wirl. With the electrode wear reduction advantag~s described herein, the level of ~wirl introduced by these holes can be greater than ha~ heretoore practi~able resulting in a better cut quality than has heretofore been attainable. The swirl ring i~ shown in a tight fitting, gas sealed relationship to ~he elec~rode. It will be understood, however, that the swirl ring is often mounted in a support member and does not itself seal to the electrode. More generally, the particular construction detail~ o the torch body ~nd arrangements for mounting these elements directing gas and cooling fluid flows and providing ele~trical connections can and do take a wide variety o~ forms.
A ~egative power lead 34 surrounds the upper end of the electrode 22 to mak~ a good electrical connection. A r~taining cap 36 threads onto a curren~
ring 38 that forms the upper end of the torch body }4. An in~ulatsr sleeve 40 separates and electrically isolates the curr~nt ring 38 from the negatiYe pow~r le~d 34. The retaining cap has a lower lip 36a which 208~9 engages a flange 26a on th~ nozzle in an abutting relationship. The retaining cap when tightened onto the ring 38 captures and replaceably secures the nozzle against the swirl ring. In ~n alternative construction not sho~, it can also capture and ~ecure the swirl ring between the ~ozzle and another internal support structure within the torch body. In the coniguration ~hown, the retaining cap 36 also, in par~, defines a gas plenum chamber 42 tha~ acts as a local supply to the swirl ring 28 and ~he plasma chamber 32. A flow 44 of plasma gas passes ~hrough an inlet tube 46 which penetrates the current ring 3~ to ~eed the gas to ~he plenum chamber 42. A solenoid valve 48 such as Model No. AFP33183 manu~actured by Automatic Switch Company is secured in the inlet tube, preferably at a point closely spaced from the torch body. Control signals to the valve over lines 48a,48a open and close the valve to regulate the flow rate of the gas to the torch. In a pilot arc mode of operation, where a pilot arc is drawn between the electrode and the nozzle thrvugh the gas in the plasma chamber, the nozzle acts as an anode and the aap 3~ ¦
and current ring 38 form a pilot arc current return circui~.
In operation, plasma gas 44 flows through the solenoid ~alve 48 and the i~let tube 46 i~to the plenum chamber 42. From there, it ~lows through the ga~ distribution holes 30 in the swirl ring 8 into the plasma arc chamber 32 and finally out of the torch through the nozzle orifice 16. When the torch i~
operating in the nontransferred pilo$ arc mode, a power supply 50 prov;des current to the torch at an ~ ~ v ~ 9 appropriats voltage to i~itiate the pilot arc and then maintain it. The power supply can be any conventional regulated D.C. supply and includes a p.c. board or the like which controls the operation of the power supply and other components o~ the torch system such as ~low control valves.
With reference to the complete plasma arc cutting torch system 52 depicted in Fig. lc, ~he complete current path in the nontransferred pilot arc mode is from the negative power supply terminal 50a, through lead 5~, ~he negative power lead 34, electrode ~2, a pilot arc pl~sma 56 (Fig. lA), the nozzle 26, the retaining cap 36, the current ring 38, a pilo~ arc return lead 58, a closed pilot arc ~witch 60, and a power supply positive terminal 50b. When the torch 12 is lowered toward the workpiece 20, the arc transfers ~o the workpiece as the ionized plasma jet 18. This allows ~ome current to flow from the workpiece through a ground lead 62 back to the power supply positive terminal 50b. When ~his transferred current path is established, the pilot arc switch opens ~nd the torch is cutting the workpiece. In this transerred or cut~ing mode, the current path is rom the power supply negative terminal 50a, the lead 54, negative pow~r lead 34, electrode 22, the plasma ar~ or ~et 18, the wor~piece 20, the ground lead 62 and the power ~upply positive ~erminal 50b.
An operator sets a desired gas flow or pressure assoclat~d with full power operation in a transferred arc mode prior to initiating the pilot arc at a control ~onsole 64 which is removed from the torch itself. The console includes ga~ flow .

20814~9 regulators, gas valving, and pressure gauges. The flow and pressure values ~et by ~he operator at the console correspond in a known way to the actual gas flow-and pressure in the plasma chamber 32 prior to the pilot ar~. Starting the pilot arc heats ~he gas causing a~ increase in the gas ~emperature within the chamber and a decrease in the flow, in a manner well known in the art. A ~pical gas pressure in the chamber 32 during the pilot arc is 20 to 40 psi. For high power torches which are the particular focus of the present invention, the gas pressures are usually at or near 40 psi. The pilot arc is started by a high-frequency spark or other means, such as a contact starting technigue, all of which are well known in the art. During start up ~he plasma gas 44 flows through th~ tube 46, solenoid valve 48, plenum chamber 42, swirl holes 30, the plasma chamber 32 and out the exit orifice 16. As noted above, the swirling flow established by the holes 30 is very important in ob~aining a good quality cut and in stabilizing the arc within the nozzle exit orifice 16 -- to prevent the arc from impinging on the noæzle and gouging it.
As will be di~cussed in detail below, the operational life of the electrode can be increased further by utilizing a novel start up procedure and the apparatus illu~trated in Figs. 4 and 5 and the novel swirl ring and asæociated gas flow control functions described with reference to Figs. 6-8.
Also as noted above, the torch b~gins autting wh~n it is brought close to the workpiece so that the arc transfers to the workpiece and the controller opens switch 60. After transfer, in normal operation 2 ~ 9 the controller increases the current level to a predetermined value for cutting. This increase in current also resul~s in an increase in the heating of the plasma ga~, a further increase in the gas pressure in t~e plasma chamber, and a further decrea~e in the gas flow out of the nozzle exit orifice~ The maximum recommended current levels Yary greatly among different torches and applica~ions, with currents in the range of about 20 to 200 amperes D.C. being characterized as low currents and those 200 and above being high currents. A typical current level for a water cooled cut~ing torch used ~o cut plate steel is 260 amperes.
A discovery of applicants is ~hat a significant loss of material (wear) of the Plectrode during its operation occurs not during the actual cutting, but rather when the current to the arc is shut off. While the mechanisms for this wear are not fully understood, there is evidence that the electrode becomes molten, at least in part, during operation and that on ~ut off of the electrical power wear i5 rela~ed ~o a complicated interaction between the molten surface(s~ of the electrode and the flow and pressure of the plasma gas through the plasma ~hamber.
The ~wirl ring and associa~ed gas flow control process of the present invention builds o~
applica~' discovery that a control of the plasma ga~
~low to the pla~ma chamber in conjunction with a controlled electrical shut off to reduce electrode wear substa~tially. In its simplest form, thi~ wear reduction involves a total shut off of the pla~ma gas flow to the chamber 30 just before (1) a total, step .. . . . . . . . . . .. .. . . .

2081~g function shut off of the arc current or (2) at the same time ~he arc current starts a gradual shut of~, but also just ~efore a total shut of of ~he arc cuxrent. This efect is achieved by closing.the solenoid valve 48 just pxior to the total 6hutting of proc~ss of the arc curr~n~. The preferred process for ~hutt;ng off the curren~ is a controlled linear decreasing ramp which follows the decreasing mass flow. The ~iming of these shut offs is, however, critical. If the gas flow is allowed to decrease rapidly, there is a significant diminution of the swirling plasma flow that stabili~es the arc.
Therefore the arc can and will attack and damage, or even destroy, the nozzle in a very short time. ~n the other hand, if the shut-offs are too close in time, the gas flow and pressure in the chamber 30, which decay in a generally exponential manner with the ~alve ~8 shut, e~hibit little ahange and wear occurs to about the same exten~ as if the valve 48 was left open. The gas flow shut off continues through the arc current shut of, and ~hereafter.
The timing of the gas and current shut off processes are illustrated by the timing diagrams of Fig. lD. The three lefthand graphs show the control signals ~whether a voltage signal, current signal, or otherwise) as a function of time. The first lefthand gr~ph shows a control signal (applied over lines 48a,~8a) to the ~olenoid valve 48 changing it6 state at a time tl ~the ~ignal goes fro~ a "1" ~ate to a "O" ~tate which are indicative of either a digital switching or an analog change sufficient to prodwe the desir~d change in state of the valve 48~. Th;s .. . .. . . . . .. . . . ........ .

208~

ch~nge in the control signal clos~s the valve at tl. The second lefthand graph shows a control signal for the arc ~urrent, a signal generated by the controller of the power supply 50. The arc current : control signal changes its ~tate, again shown as a change from a "1" state to a "O" state, at a time t2 which is after time tl. The gas flow is ~herefore cut off before ~he arc current by a tims i~t~rval ~t ~qual to the difference betwe~n t2 and tl. The third lef~hand graph shows an alternative control seguence for ~he arc current cut-off process. In accordance with this invention the arc current control signal changes it6 state, again shown as a change from a "1" ~tate to a "O" state, at a time tl which is ~he same time as the valv~ control siynal. This change commences ~he operation of the controller to produce a ramp down of the arc current as shown in the lowermost righthand graph of Fig. lD.
The righthand graphs in Fig. lD show: 1~ the ~as flow rate through the plasma chamber 32; 2) t~e arc current shut off proce~s as a sudden step off ~t ~2; and 3) an alternative arc current ~hut o~
proces~ as a gradual linear ramp down from full current at tl to a minimum ~ustainable current at t2 when the arc current snuffs out to zero. These .
graphs ar~ also presented as a function of time and for the same periods of time as the corresponding lefthand grap~s. Aft~r closlng the valve 48 at tl, th~ gas ~low fall~ steadily. A yenerally linear fall off i~ flow i~ shown, but the relationship is actually more complex and the curve is in fact generally exponential. The important factor is that the gas 2~8~
--lg--1OW value falls substantially over the internal at so that a) it is at a comparatively low value at t2 when the arc current is shut off, as illustrated by the second righthand graph o Fig. lD, or b) it i~ a comparatively low value at t2 following the current ramping down illustrated by ~he third righthand graph of Fig. lD. The pr~sently preferred arrangement i8 to use a non-vented ramp down of the gas flow a~ shown in the upper righthand graph in combination with an arc current ramp down as shown in the lower righthand graph tha~ follows the change of the gas flow over the ~ime interval ~t~ This preferred mode o operation seems to produce the least wear despite the fact that a lower arc current changes the gas density in a manner that produces an increased gas flow rate through the plasma chamber -- other factors being constant. The swirl ring described below with respPct to Figs. 6-8 addresses the problem of this increased gas flow rate.
While the precise value of ~t varies with each torch and the particular operating parameters, for mos~ low current plasma arc cutting applications a ~t of 500 milliseconds or less has been found to be the right timing to reduce electrode wear. For ~he MAX 2ao brand torch, a ~t of roughly 250 to 300 milli~econds without venting and with a following current ramp down has been found to be optimal. In the operation of a ~AX 100 brand torch after 120 cut cycles using thi~ ga~ flow/arc current control there is a pit depth (wear) of about 0.005 inch, whereas normal operation without these controls produces a wear depth of about 0.025 inch in the same electrode 208i ~9 insert 24. This wear reduction translates in~o an electrode life which is five times the best value that has ever before been attainable. This gas flow~arc current control may also allow, on average, torches to be operated at powers in ~xcess of their conventional rati~gs.
Note that at t2 there is still a residual gas flow even though the gas feed is cut off at tl.
This ensures that ~ntil and at current cut-of~ there is a sufficient flow in the chamber to stabilize the arc and prevent nozzle damage. Also, there is a hrief surge in the flow after cut-off of the current. This is believed to reflect a sudden cooling of the gas i~
the absence Gf the arc and a rapid out-~low of ga~
from the torch driven by the gas pressure in the plasma chamber and the sudden change in the properties of the gas after the arc is extinguished. This rapid out-flow can be analogized to an explosive decompression. It is particularly troublesome i~ the high power torches.
This out-flow phenomena ~ gests that while the gas flow to the torch can be reduced over the interval at by reducing the flow 44 to the plasma tor~h, that a complete closure of the valve 48 is preferred since this closure upstream of the plasma chamber dampens the ~trength of the ~low surge and limits the total volume of the flow when the curr~nt is cut-off. As already ~tated, it is also contemplated to reduce the current at tl, e.g. by ramping it dow~ over the interval ~t rather than having an abrupt shut off at t2. It is also contemplated ~o allow a reduction in the overall !

2081~59 current prior to tl, or a~ter t1, i.e., prior to or after closing the solenoid valve.
Figs. 2A-2C show the plasma arc cutting torch 12' incorporating an alternative gas flow control arrangement to reduce elec~rodP wear, like parts in the Fig. 2A-2C embodiment having the same reference numbers as in Figs. lA-lC, but with a prime. The ~tructure and mode of operation of the torch 12' and torch system 52' is the same as described above with respec~ to the Figs. lA-lC embodiment, except for ~he addition of a vent tube 66 and an associated solenoid valvQ 68 connec~ed in the vent tube to open and close it. The tube 66 penetrates the current ring 3~' and is in fluid communication with the plenum chamber 42'. A control signal from the controll~r carried over lin~s 68a,68a operates the valve 68. I~ thi~
e~bodiment, when the sol~noid valve 48 is c~osed at time tl. Th~ vent valve 68 i~ opened. Because the vent tube 6B is open at its end ~6a at atmosphere, or to some other lower pressure region such as a vacuum chamber, opening the valYe 68 causes the gas flow ~nd presæure in the plenum and the plasma chamber to decay more rapidly than the decay of the Figs. lA-lC
embodiment. This allow~ the current to be shut off more quickly after the gas flow is cut off at time ~ has been discovered, however, that timing i8 Yery important in this configuration. æînce by venting an alternative flow path i5 establi~hed, the flow through the nozzle can go to low values and cause the plasma to become unstable very quickly. I~
general, when v~nting is used the flow alteration preceeds the arc current shut off by a significantly . . . ... .. . . - . - ~ - -2~8~3 short period of time and without venting. With this venting, the interval ~t can be reduced from about 250 milliseconds to less than 5 milliseconds when operating a MAX ~U0 brand torch with air. This may r~duce the likelihood of nozzle ~amage caused by a destabilized arc. It is also contemplated that Y~lves 48' and 68 can be combined in a single ven~ing-type ~alve.
Figs. 3A-3D show another embodimen~ of a torch 12" and torch system 52" utilizing according to ~he presen~ invention, like parts being identified with the s~me reference numbers, but double primed.
This embodiment uti}izes the discovery that electxode wear can be reduced subst~ntially if the gas flow through the plasma ch~mber is changed not only in flow rate, but also in flow pattexn, ju t prior to current shut off. More specifically, electrode wear is reduced ~o almost negligible levels on current shut off if ~he degree of ~wirling of the gas is reduced just before cut-off. At moderate gas pressures, thi~
resul~ holds for even high ga6 flow rat~s (e.g. 120 scfh). In operation with the Hypertherm0 MAX 100 brand tsrch, negligible wear was observed when the gas flow into the plasma chamber w~s radial (no ~wirl~ and the gas pressure in the cha~r~er was below 30 psi.
While a perfectly radial flow and moderate to low gas pressures produce the best results, this arrangement al~o provides reduced electrode wear with less than a perfectly radial flow and at increased gas pressures.
In this embodiment a major ~oncern once agaîn is destabilization of the plasma arc in the absence of a ~wirli~g flow. ~pplicants' solutio~ is to use a 2~81~

swirling flow, and then suddenly switch to a radial flow, with substantially no interruption of the overall flow rate, immediately before ~urr~nt cut off. The ~orch 12" and system 52" accompli~h this mode of operation.
The torch 1~" has generally the same construction as the torches 12 and 12', except that the torch is ~erviced by two ~eparat~ ga~ feed lines, each with its own solenoid valve, which feed separate plen~m cha~bers and in ~urn feed separate, independent inlet holes in the swirl ring 28" or an equivalent structure. In the preferred form shown, there is a first gas flow 44c which passes through the inle~ tube 46", the valve 48", an annular plenum chamber 42", the inlet holes 3G", the plasma chamber 32" and out the exit nozzle orifice 16". This gas 10w path provides a swirling ga6 ~low for cutting that produces a good quality cut and stabilizes the arc. The swirl is established by the holes 30" which are radially ofset a~ i~ best seen in Fig. 3B. The plenum ~hamber 42" i~
defined by the ame components as in previous embodiment except that the current ring ha~ an annular downwardly extending wall 38a" and a flange 38b" at the lower ~dge that abu~s a step recess in the swirl ring 28" in a ga~ tight seal. ~It will be understood that the seal can b~ secured wi~h 0-ring~, a labyrinth seal, or any conventional gas seal that also allow~
the swirl ring to be disassembled from the torch as necessary.~ The wall 38a" and flange 38b" separate and isolate from one another the outer plenum chamber 42" and an inner plenum ~hamber 42d".
A gas flow 44d passes through an inlet tube .

`~` 2~814~

46d", a valve 48d", the plenum chamber 42d", inlet holes 30d", the plasma arc chamber 32" and out the exit nozzle orifice 16". This second gas flow path for the flow 44d uses inlet holes 30d" in the swirl ring that are generally radially directed as is best ~een in Fig. 3C. The gas flow through ~he plasma chamber is therefore generall~ axial (downwardly as shown~ through the chamber 32" to ~he exit 16"; there is substantially no swirl.
In accordance with this flow pattern al~ering process for electrode wear reduction, at a predetermined bu~- very brief interval ~t ~efore the current to the torch is cut off, the controller clo~es the valve 48" for the flow 44c and opens the val~e 48d" for the flow 44d. The interval ~t for a MAX
100 brand torch operating with a reacting gas is typically less than 500 milliseconds. This change in flow pattern, with no other changes in flow or current parameters, has also been found to provide dra~atic reductions in electrode wear. However, this e~bodiment can be combined with the mass flow rate reduction embodiment described above with respect to Figs. lA-lD and 2A-2C. For example, the current level can be ramped down after tl.
Fig. 3D shows a 8uitable system 52" for practicing the invention in this altered flow pattern mode. A control con~ole 64" remote from the torch and therefore the substantial electromagnetic interference produced by the torch, controls the gas flow 44c. A
like conssle 64a" controls the flow 44d. I~ practice the consoles 64" and 64a" can be a single unit.
While various time periods bave been 2~8~9 ~uggested above for ~t, the optimal interval will de~end on the specific torch, it~ applications, and related parameters. In general/ Qt is a function of the t~pe of ga~, the current level, sîze of nozzle orifice, inlet flow area of swirl ring, the gas pressure, the yas 10w rate, the gas flow pat~ern, and the physical separation between the solenoid valves in th~ inlet and ~ent tubes and the plasma. The separation is preferably less than 12 inches for the MAX 100 brand torch. This separation helps to control delay~ and unintended variations in flow parameters due to the presence of a large fluid mass upstream of a plasma chamber and downstream of th~ valve. The values for an acceptable interval ~t can readily be determined empirically. Also, while the invention as described ~hus far h~s focused on the alteration of the gas flow just prior to cut off, it should be understood that the altered condition continues through electrical cut-off and for a brief period thereafter. However, the flow usually ceases entirely very shortly after cut-off, whether due to a clo ing of the æolenoid valve in the inlet tube which eventually brings the flow through the plasma chamber to zero, or through a clo6ing of the valve 48d" i~ the l'radial" gas low path to the radial hole 30d" in the Fig. 3A-3D embodiment.
Fig. ~ ~hows yet ~nother embodiment of a torch 12'" and a torch system 52'", like parts being identified with the same reference number, ~ut triple primed. Thi~ embodiment is like the embodiment ~hown in Figs. lA-lD ew Ppt that it has two sources of plasma gas connect~d via tube 7B and on-off ~o~enoid .. . ..
.

2081~9 valve 48'" to ~he inlet tube ~6'" which in turn directs the gas to the interior of the torch 12'" and its Elasma ohamber. A preflow gas flow 4~f flows through the preflow remote eonsole 84 including an orifice 84a, a pressure gauge and a 10w meter. The preflow passes through conduit B2 and a pre~low on-off ~olenoid valve 80 into a branch condui~ 7~a and then i~to a gas feed tube 78. An operating plasma gas flow ~4'" 10ws through the operating flow remote console 74 including an orifice 74a, a pressure gauge, and a 10w meter. The operating gas flow ~4'" then p~sses through a condui~ ?~ and an on-off solenoid valve 70 into a branch conduit 7~b, and then into the gas feed tube 78. The two soruces of plasma gas are connectad in parallel ~ia branch tube 78a and branch 78b to the feed tube 7~.
A further discovery of a~plicants is that signiicant electrode wear can occur on start up as well as cu~ off. In particular it haæ bee~ found that al~eration of the gas flow to the torch on start up can make substantial improveme~ts in the electrode life even as ~ompared ~o the substantial improvements achieved with the cut off controls described above with respect to Figs. lA-3D. Specifically, the s~ar~
up procedure and apparatus described below in detail have been found to double the life of elec~rodes when aiso using the cut-off procedure and apparatus described with reference to Figs. lA-3D. A~ electrode life about 10 times than that obtainable us;ng prior art procedures and e~uipment i~ readily obtainable.
The æolenoid valves op~rat~ automatically in response to control signals produced by the power :

2~8~9 ~27-supply and directed to the valves over lines 86a, 86b and 86c. The v~lves 48'", 70 and 80 can also be solenoid feed valves of the type described above with respect to valve 48. The 1OW ori~ices 84a and 74a can be adjusted manually or automa~ical}y.
In operation, to start the torch 12'"
according to the present invention, a preflow 4~f of the ioni~able gas is directed from the source to the torch. The preflow is at a ~ignificantly lower flow rate and pres~ure that the gas flow rat~ and pressure associated with full power operation when ~he arc has transferred to a workpiece.
With reference to Fig. 5, the start of the plasma torch is indicated at t9. A control signal initiated by the operator causes signals to be sent over lines 86a and 86c to open valves 48'" and 80 respectively, but maintain valve 76 in a closed condition. All of the gas flow to the torch therefore passes through, ~ld is limited by, the adjustable orifice 84a. It i~ set to establish a ga~ ma~s ~low rate that i~ (i) sufficient to initiate and sustain a pilo~ arc in the torch 12'", but also ~ii) ~uficiently below the full operating value in the transferred arc mode that the improved wear characteri~tics of the pres~nt invention are observed. By way of illustration, a ~ull flow rate can be 40 ~cfh for a Hxpsrtherm~ ~X 100 brand torch and the preflow can be 24 scfh, or about 60~ of the full flow. The precise value ~or the preflow will vary depending o~ the ~actors such as thcse enum~rated a~ove with respect to the timing of the alteri~g of the gas 1OW on cut offt but it will be within the . .

2~81~9 -2~-functional "end limits" given above, that is, being able to ignite a pilot arc while exhibiting the wear improvement of the magnitude desGribed herein. The middle graphs in Fig. 5 raflect valves 48'" and 80 opening at to. The top graph reflects an increase in the gas pressure in the plasma chamber as a result of the preflow through these val~es.
Time tl in Fig. 5 re~lects the time when the arc transfers ~o the workpiece and the torch begins operating in the transferred arc mode. The time interval to to tl is pr~ferably suficient to produce a stable preflow. Four seconds i~ a typical value for this interval. Just prior to tl there is a small step increas~ in the arc current, as illustra~ed in the second graph from the top in Fig.
5. This current is a standard pilot arc current. The pilot arc is ignited after the preflow is stabilized and is maintained for a brief interval before tl, a typical value being 20 milliseconds.
A~ tl, the transer of the arc generates another control signal ~ent over line 86b ~o ope~
valve 70 as re~lected in the third graph from the top in Fig. 5. The power supply also begins to increase the arc current at tl toward a desired full operating value. At the ~ame time the control ~ig~al 86c causes the preflow solenoid valve 80 to close.
The opening of valve 70 allows an increased yas flow ~o the ~orch as reflected by the rise in ~he gas pressur~ at the inlet to the torch as ~hown in the top graph in Fig. 5. The orifice 74a sets the maximum flow ~o the torch. Due to the lead from the valves to the plasma chamber, the increase in the gax flow and 2 ~

- :29--pres~ure is not a sharp step function, but rather a smooth ramping up, as ~hown. The increase in the gas flow i5 preferably in coordi.nation with the increase in the arc current, also as ~how~. The curre~t and gas 10w need not increase preci~ely in coordinatlon, bu~ the flow mu~t be increased sufficiently ~o as to sustain the ~ran~ferred arc at the current level then prevailing. Th~ flow must also æwirl and have a sufficient ~low rate ~o guide the plasma je~ and contain i~ against attacking the torch itself. Thi~
valve and conduit arrangement also lends itself to the use of one type o gas a~ a pre~low gas and another type as the plasma gas in a ~ransferred arc mode.
Preferably a less reactive gas such as nitrogen or argon is used for the preflow. The switch over from a preflow to an operating flow attendant opening valve 70 and closing valve 80 on arc tra~fer allows the introduetion of a more reactive type of gas such as pure oxygen or air. Alternatively, the preflow can be air and the operating flow can be oxygen. When this preflow and ramp up start procedure i~ used in combination with the cut-off procedures described above, the electrode life can readily and reliably be extended by a factor of about ten.
While the ~tart up f~atures have been described with respect to a change i~ the low rate produced by a valve networ~, ~he beneficial effects of an axial gas ~low evident on ~ut off also apply to start up. A swirling flow i8 necessary to control the arc during operation, but at least during the pre10w phase when the pilot arc ~s ignited, the flow pattern can be axial to reduce wear, and then ~witched over to `
2 ~ 9 a swirling flow on transfer at tl. This change in flow pattern can be combined with an increase in flow rate before, at, or after the switch over at tl.
The apparatus shown in Figs. 3A-3~ can be used for thi~ ~low pattern alteration on start up as well as on cut of.
To end a cycle of operation with the Fig. 4 embodiment and to utilize the cut off advantages described above, the valves 48'" and 7G can be close at t2 causing a decay in the gas pressure and flow.
There is a corresponding decrease in the arc current, all as described above with respect to Figs. lA-3D.
At time t3 the cycle of operation is complete and the torch is off.
Figs. 6-8 shsw a novel swirl ring 90 according to the present invention which replaces the conventional sw;rl ring 38 shown in Figs. lA, lB, 2A, 2B, and 3A ~ 3C. The swirl ring 90 i6 particularly designed for use in conjunction with the gas 1Ow/arc current controls described above with reference to Figs. lA - ~. It is particularly effective in reducing electrode wear in high power torches such as the Hypertherm~ HT400 and PAC 500 brand torches where the gas flow~arc control procedures described abo~e have proven to be less efectiv~ than with low~r power torches such as those rated at up to 200 amperes.
It has been discovered that for these high power torches the gas out-flow on cut-off is sufficiently strong and of sufficiently long duration to produce an unacceptable degree of electrode wear despite the use of the apparatus and pro~esses discussed hereinabove, despite the location of th~ gas 2 0 ~ 9 cut off valves 48,48' and 48'" close to the torch, a~d despite operation with the gas flow being totally shut off ~ust prior to cut of of the arc current, as oppo~ed to merely being significantly xeduced in flow rate.
The ~wirl ring 90 has been found to overcome these problems in high power torches and thereby extend the electrode liPe to the same extent as described above with respect to Figs. lA - 4. The swirl ring 90 is a generally annular member formed of a conventional structural material used conven~ionally for swirl rings. The pref~rred material is lava rock, whieh is machinable ceramic. Th~ ring 90 has a generally rectangular cross section, as ~hown in Fig.
6, with an upstream face 90a in fluid communication wi~h a gas passaye 46"", or an eguivalent gas conducting tube or chamber formed in ox secured within the torch body 12"" (like parts in the Fig 6-7 embodiment being identified with a quadruple prime3.
The gas flow 44 to the torch flow is substantially unimpeded (except for the action of the various valves and meters descri~ed hereinabove) to the upper ~ace 90a. The ring 90 has a lower or downstream face 90b open to the plasma arc chamber 32"", de~ined mainly by the electrode 22"", the no~zle 2fi"" and the swirl ring 90 i~self, as shown. An inner lateral face 90c i~
also open to th~ plasma chamber 32"". ~n outer lateral face 90d abut~ a ~urroundi~g current ring 92 th~t is in electrical connection with the ~ozzle 26"".
A principal feature of the ~wirl ring 90 î~ a pre-chamber 94 formed in the preferr~d form shown as a 2081~

central annular recess in the outer face sod of the swixl rin~. Ano~her principal feature is a 6et of equiangularly spaced choke holes 96 driiled in ~he ring ~0 and extending from the upper face 90a to the pre-chamber 94 to provide a fluid passage for ~h~ flow 44 from the passage 46"", or a comparable ~tructure, to the pre-ch~mber. It i~ lmportant tha~ ~he choke holes 96 are of a number and size that together they conduct a sufficient flow o the working gas to the plasma arc chamber to sustain and stabilize the arc during steady state operation, but at the same time ~hey are sufficiently restrictive to this flow tha~ a significant drop appears in the gas pressure across the holes 96. By way of illustration, but not of limitation~ where the pressure in the plasma chamb~r during teady state operation o a high power torch is 40 psi, the pressure drop across the holes 96 is about 32 psi. Also by way of illustration only, for a nozzle ring sui~able for a HT400 brand plasma arc torch manufactured ~y Hypertherm, Inc., there are 8iX
choke holes, spaced equiangularly, each having a constant internal diameter of about 0.018 inch and a length of about 0.136 inch.
-~ The gas entering the swirl ring and held i~
the pre-chamber 94 exit~ to the plasma chamber via set of swirl holes 98 drill~d at an angle in the lower face 90b of the ring and a s~t of radial holes lOQ
drilled in ~he inner lateral face 90c. The swirl holes are equiangularly spaced to facilitate a uniform ~lvw distribution of the swirling gas i~ the plasma chamber. By way of illu~tration but not of limitation, there are twenty four swirl holes 98, each 20~1~59 having a diameter o 0.0~5 inch and angled at 20 from the vertical to introduce a tangential swirling componènt to the gas flow exiting the swirl ring. The radial holes also extend from the pre-chamber to the plasma chamber to ~et up an axial flow of gas ~o the plasma chamber. The number and dlmçnsions of the radial holes 100 is such tha~ they contribu~e som~
axial flow in~o ~h~ plasma chamber during steady state operation, but it is introduced upstream of the swirling flow and its flow ~attern is overcome by the swirling flow in normal operation. The radial holes aid in distributing the gas uniformly through the plasma chamber, both by contributing ~ gas flow and by placing the upper end of the plasma chamber at the same gas pressure as the lower end, ad~acent the swirl holes 96. By way of illustration but not of limitation, the radial holes 100 are four in number, equiangularly spaced, and have at their minimum diameter the same 0.018 inch diameter as the choke holes.
In the preferred form illustrated and described, the swirl ring 90 reguires three seals to provide fluid isolation of the three ch~nbers -- the inlet ~6"", the pre-chamber 94 and the plasma chamber 32"" . A f irst o-ring 102 is seated in a shoulder recess 90a' at the upstream face of the swirl ring; it blocks a 1OW of the gas directly from ~he gas ~xit from the torch body to the plasma chamber. The o-ring 102 abuts and seals to a portion 40'" of the torch body. A second o-ring 104 ~eated in an annular reces~
formed on the out~r face 90d of the swirl ring blocks a direct gas flow from the torch body to the 2~8~9 ~3~-chamber. The o-ring 104 abuts ~nd seals to the riny 92. The o rin~s 102 and 104 force the gas p~ssing through the torch body ~o pass n the.pre-chamber 94 also. A third o-ring 106 ~n an annular recess 90f ~n ~he downstream face hlock any leakage o~ gas exiting the ~wirl ring ~ - ----- - - ~han through the plasma chamber 3~ o the exit 16"". The o-ring 106 abuts and seals to the This swirl ring construction places a cant flow restriction, the choke holes 96, ver~
o the plasma chamber. On cut-off, a~ the cools and there is a sudden out flow of gas, - - .--.~ _ - ssure drop at the choke holes restricts the w of the gas held in the torch body and the .ely short gas conduits ~panning the di~tance he control valves 48,48' and 48'" and t~e ~- - ing. (~he closest upstream valve is closed o the arc current cut-off to assist in a rapid - - . - . _ of ~he plasma gas flow in the plasma gas ) The pre-chamber 94 therefore acts as a ~cal reservoir of gas to supply the plasma - - during the milliseconds between a ~hut off of - . -----:----~- -- flow and a total cut off of the arc current.
= important since the simple expedient o ==================~g off the ga~ flow totally prior to arc cut-off - ~-r ~ ng the residual gas ~o b~ used up in ~3 allows the arc to become destabilized and .Ihe nozzle. The pre-chamber ~ therefore ~ized _ y a volume of gas to the plasma chamber i~ the t between the gas shut off and the arc .-shut off suffi~ient to ~aintain the arc in a 2~81~9 stabilized condition, but not sufficient to exhibit a degree of electrode wear ~haracteris~ic of prior art torches. By way of illustration, but not of limitatlon, the pre-chamber 94 has cross sectional dimensions of about 0.114 inch by 0.1~7 i~ch with an I.D. of about .764 inch and. an O.D. of about .996 inch for use with the HT400 brand torch and with the illustrative hole dimensions given above.
Fig. 8 is a graph illustrating the gas control function of the swirl ring shown in Figs. 6 and 7 when also used with the gas flow and arc current controls on start up and cut-off discussed above or a Hypertherm~ MAX 200 brand pla~ma arc torch. There is a preflow o~ plasma gas on start up to bring the gas pressure in the plasma chamber to about ~ psi.
Firing a pilot arc raises the pressure to about 12 psi. On arc ~ransfer to a workpiece, the plasma gas pressure increases further as the valves are op~ned and then arc current increase~ to its ~ull operational value. At the completion o~ an operating cycle, the arc current is ramp~d down (~ cut off) and ~he plasma gas flow is shut off at the upstream control valve 4B'". The arc is then stabilized by ga~ drawn out of the pre-chamber through the holes 98 and 100, with the choke holes 96 re~tricting an inflow o gas srom upstream conduits, passages or cham~ers in the torch body. Preferably, a8 ~how~, the ar~ curr~t is shut off completely when the plasma ga~ pressure ~alls to abou~ 4 p~i. Stated in other words, the volume of gas available for a rapid expansion ~h~n the ar~ ~uts off and the plasma cools is guite small. mi~ allows the gas control/arc current contro1 procedures 2Q81~59 described above to operate and to extend the electrode }ife, even for h~gh current torches. The swirl ring and associated gas flow control features also enhance the electrode life in 1QW to ~edium power torches.
Another significan~ aspéct of the ~wirl ring 90 according to the pr~sent invention is that it also distributes the ga~ very uniformly due to the ~ize, number and location of the gas conducting holes 96, 98 and 100 formed in the ring. This is in contrast, for example, to a possible expedient of simply creating a restriction in the gas outlet from ths torch body ~o the plasma chamber. One restriction was found to create non-uniform, high-velocity gas je~s at ~he outlet of the restriction which produced a highly uneven wear on ~he electrode and reduced the ~uality of the cut in the workpiece. A characteristic of the present invention is that the plasma gas flow pattern is substantially indspendent o~ the choXe holes; it can be altered using standard techniques u6ed for conventional plasma arc cutting torches.
There has been described a gas flow control process and novel swirl ring construction for reducing the wear on an electrode of a plasma arc torch, particularly ~ high power cutting torch, bu~ in gsneral for all types of plasma torches, e.g. ones f~r welding, spraying or other applications. The invention, in any of its embodiment~, can reduce th~
wear ~hat presently occurs on all electrodes to an extent that the life of the electrod~ is at l~ast doubled and can be as much as ten times or more. The inventio~ allows a given torch to be operated at increased power levels and with reactive gases. These .. ,, .. , ~ .. . . . .

2~81~9 advantages can be achieved with no dimunition of cut quality, and using standard electrodes and nozzles.
In fact, due to the discovery that the swirling 10w does not adversely affec~ ~he electrode wear during cut~ing, and knowing that a strong swirl yields high quality c~ts, the swirl strength can now be increased in torches to improve cut quality. Moreover, existing plasma arc torches and complete torch ~ystems can be readily modified to use ~he present invention.
While this invention ha~ been described with reference to its preferred embodiment~, it will be under~tood that various modifications and alterations will occur to those skilled in the art from the foregoing detailed description and the accompanying drawings. For example, while the inve~tion has been described with respect to a swirl ring containing the choke ~oles and a pre-chamber, these features could be built into the torch body, components forming the torch body, or other components such as even a portion of the nozzle itself or a flange on a retaining ~ap.
The flow control concept of the invention, regardless of the details sf its implementation, i~ that a plasma gas flow restriction is placed in the plasma yas ~low path immediately preceding th~ plasma chamber and a cmall supply of gas is a~ailable downstream of the restriction, but upstream of the plasma chamber.
These and other modifications and variations are inte~ded ~o fall wi~hin the ~cope of the append~d claims.
Wbat is claimed is:

.. . . .. .. .. . . .

Claims (14)

Claims:

1. A process for controlling wear of an electrode in a plasma arc torch at the termination of a cycle of operation where an arc current and plasma gas flow through the torch are cut off, and where the electrode is mounted within a torch body opposite and in a spaced relationship to a nozzle having an exit orifice for a plasma arc, the electrode and nozzle defining a plasma chamber, the torch body including means for directing a flow of an ionizable gas through the torch body to the plasma chamber, and the torch body having a conductive path for an arc current to produce a plasma arc in said gas flow eminating from said electrode both as a pilot arc to the nozzle and as a transferred arc to a workpiece at a full operating arc current value, comprising creating a pressure drop in said flow of ionizable gas immediately upstream of said plasma chamber, said pressure drop being sufficiently low that the gas flow into the plasma chamber at the full operating value produces a gas pressure and gas flow pattern in said plasma chamber that stabilizes the arc, and said pressure drop being sufficiently large.
that at said arc current cut off the gas pressure and the gas flow through said plasma chamber decays rapidly in coordination with the arc current cut off.

2. The electrode wear control process of claim 1 wherein said creating of a pressure drop comprises restricting the gas flow to the plasma chamber within the torch.

3. The electrode wear control process of claim 1 further comprising the step of producing a local gas chamber in said gas flow between the point of said pressure drop in the torch and said plasma chamber.

4. The electrode wear control process of claim 3 further comprising distributing the ionizable gas flow from said local gas chamber to said plasma chamber in a flow pattern that is swirling.

5. The electrode wear control process of claim 4 wherein said gas distributing also produces a generally spatially uniform flow of said gas through said plasma chamber.

6. The electrode wear control process of claim 1 wherein said gas flow shutting off precedes the arc current cut off by a very brief time interval in which the gas held in said local gas chamber and the plasma chamber stabilize the arc until it is extinguished.

7. The elctrode wear control process of claim 6 wherein said arc current cut off is ramped down.

8. A gas control swirl ring for a plasma arc torch having a torch body that mounts a nozzle and an electrode at one end of the body in a spaced relationship that defines therebetween a plasma arc chamber where an arc current conducted through the torch body ignites a plasma arc in a flow of an ionizable gas directed through the torch body to the plasma arc chamber via a gas inlet passage produces a plasma arc that transfers to a workpiece via an exit orifice in the nozzle during operation in a transferred arc mode, comprising a generally annular body having an upstream face in fluid communication with said flow of ionizable gas in the torch body and a downstream face in fluid communication with the plasma arc chamber, a gas chamber formed in the body, a plurality of gas inlet passages extending from said upstream face of the body to said chamber, said passages being sized to restrict the flow of ionizable gas to the chamber and thereby produce a pressure drop in the gas flow, and a plurality of gas distribution holes formed in said body extending between said chamber and the plasma arc chamber to produce a swirling, generally uniformly distributed flow of gas in said chamber.

9. The swirl ring of claim 8 wherein said gas chamber comprises an annular recess formed in the outer surface of said body between said upstream and downstream faces.

10. The swirl ring of claim 9 further comprising an annular first sealing recess formed in said outer surface between said upstream face and first annular recess.

11. The swirl ring of claim 9 further comprising annular second and third sealing recesses formed in the upstream and downstream faces of said body.

12. The swirl ring of claim 8 further comprising a plurality of radially directed holes formed in said body extending from said gas chamber to said an inner surface of said body extending between said upstream and downstream faces and in fluid communication with the plasma arc chamber.

13. The swirl ring of claim 8 wherein each of said gas inlet passages and said gas distribution passages are mutually equiangularly spaced and are at least four in number.

14. The swirl ring of claim 8 wherein the gas pressure in the plasma arc chamber during operation is about 40 psi and said pressure drop is about 32 psi.